After revision of the Building Standards Law in 2000, it became possible to use various materials and structural methods, if they satisfy certain performance requirements with respect to fire resistance. Although refractory structures were traditionally restricted to those made of incombustible materials such as steel constructions and reinforced concrete constructions, it is now possible to use a flammable material such as wood, if it is shown to have a particular fire resistance. Wood, a material renewable by afforestation, is a building material that is environment-friendly and can be used as a means for reducing carbon dioxide gas. Accordingly, it is desired to actively expand domestic woods, a resource abundant in Japan.
Under the Building Standards Law, the refractory building should remain undestroyed even after termination of a fire and retain the fire resistance (such as, for example, load-supporting potential, heat-insulating property, and flame-insulating property) which is expected of the members even after application of a refractory heating. In a typical method of testing fire resistance, a sample is heated in a refractory oven along the standard fire heating temperature curve (ISO-834) specified by the International Organization for Standardization and then held in the oven for a period three times as long as the heating period. The sample is examined to determine whether the sample stops burning reliably after the refractory test by analyzing the deformation and the deformation rate of the sample during the heating period. This is a strict requirement for common wooden columns and beams, and it is difficult for these wooden members to satisfy this requirement.
Various structural materials have been proposed for making a refractory structure comprising a glued laminated timber which is made of flammable woods. For example, Japanese Unexamined Patent Application Publication No. 2005-036456, Japanese Unexamined Patent Application Publication No. 2005-048585, and Japanese Unexamined Patent Application Publication No. 2008-031743 describe composite wood-based structural materials such as laminate timbers and glued laminated timbers. Such a structural material comprises, for example, a glued laminated timber having certain strength as its core material and inorganic materials such as gypsum board, mortar, and cement, various flame retardant components, and flame retardant-treated or high-density woods as the refractory layer formed around the external surface of the core material. However, for production of such a structural material, multiple boards or plates having a particular size should be prepared and high-viscosity materials should be coated on the surface of the core material surface, and thus, the production steps become very complicated. In addition, it is more difficult to produce a structural material with an arbitrary cross-sectional shape (e.g., circular cross section) and thus, production of such structural materials becomes very tedious.
Japanese Unexamined Patent Application Publication No. 2015-061969 proposes a refractory structural material comprising a refractory layer of a polyisocyanurate resin. The structural material solves the problems described above, but the fire resistance thereof is still far from sufficient.